In information recording/reproducing devices and information reproducing devices of recent years, progress has been made in increasing a capacity for storing information has been increases, increasing an information transfer rate, and reducing a size and weight of the device. In a magnetic disc or an optical disc, a disc rotation speed when performing recording or reproduction increases with the increase in the information transfer rate. A rotation axis and a center of gravity of the disc do not coincide with each other, and there is a predetermined error. Generally, an amount of the error between the rotation axis and the center of gravity is defined as mass eccentricity. The mass eccentricity causes unnecessary centrifugal force when the disc rotates. The unnecessary centrifugal force is kinetic energy, and is proportional to a square of a rotation speed. Therefore, as the disc rotation speed increases, the unnecessary centrifugal force rapidly increases in a quadric curve. Further, as the device becomes compact and lightweight, an acceleration and a position of the disc device tend to fluctuate when vibration is applied from outside. Therefore, when recording or reproducing is performed on the disc with mass eccentricity, the entire device generates self-exited vibration in synchronization with the rotation of the disc, and there is a problem that the recording or reproducing of information becomes impossible.
As a measure to solve this problem in an optical disc device in which the disc is interchangeable, there is proposed a vibration damping mechanism using an auxiliary mass and a resilient member. A sub frame functioning as the auxiliary mass is coupled with a main frame via the resilient member, and the main frame houses a disc rotation mechanism. It is disclosed that an effective anti-vibration effect is obtained by setting a resonant frequency of the sub frame to be slightly higher than a rotation frequency of the disc (see, for example, Patent Document 1). The disclosed technology is based on a principle that, by adding a dynamic damper to a specific structural object, vibration of a structural object is topically suppressed with respect to vibration input whose frequency equals to a natural frequency of the dynamic damper.
Further, there is proposed a vibration damping mechanism in which a spring and an actuator are parallely provided in addition to the vibration damping mechanism using the auxiliary mass and the resilient member. The vibration damping mechanism is configured to detecting a relative displacement between a vibrating body and the auxiliary mass, and to control driving of the actuator. However, this vibration damping device uses a large auxiliary mass (a movable mass). The actuator is controlled so as to apply a force in a direction in which a spring coefficient decreases and with an amount which is proportional to an amount of displacement of the spring (see, for example, Patent Document 2). In this regard, the spring functions as a resilient member.    Patent Document 1: Japanese Laid-open Patent Publication No. H11-328944 (paragraphs 0021, 0042 and FIG. 3)    Patent Document 2: Japanese Laid-open Patent Publication No. S60-60344 (page 3 and FIG. 2)